Stillbirth is defined as the loss of a baby before or during delivery. This loss is defined by a specific gestational threshold, typically the death of a fetus occurring at or after 20 completed weeks of pregnancy in the United States, or after 28 weeks internationally. The purpose of examining this subject is to provide factual, evidence-based information regarding how the statistical chance of stillbirth changes throughout the third trimester. This analysis relies on population-level medical data, which describes overall trends and does not predict the outcome of any individual pregnancy.
Defining Stillbirth and Baseline Incidence
The medical community uses precise gestational age cutoffs to define stillbirth, distinguishing it from miscarriage, which is a loss before 20 weeks. In the US, stillbirth is generally categorized into three groups based on timing: early (20 to 27 weeks), late (28 to 36 weeks), and term (37 weeks or later). This classification helps clinicians understand different potential causes and risk factors associated with the loss.
Nationally, the overall incidence of stillbirth is approximately 6 losses for every 1,000 total births. This rate means that stillbirth affects roughly one in 175 pregnancies that reach the 20-week mark. The statistical risk changes considerably depending on the week of gestation a pregnancy has reached.
Stillbirth Risk: Gestational Age and Week-by-Week Data
The statistical likelihood of stillbirth does not remain constant throughout the third trimester; rather, it follows a specific risk curve. In a healthy, low-risk pregnancy, the rate of stillbirth is at its lowest during the late third trimester, particularly around 37 to 38 weeks. The risk of stillbirth for an ongoing pregnancy at 37 weeks is estimated to be around 2.1 per 10,000.
However, as a pregnancy extends beyond the estimated due date of 40 weeks, the risk of stillbirth begins to increase significantly. This phenomenon is known as the post-term risk, where the chance of a loss from expectant management can surpass the risk associated with intervention. For example, the risk of stillbirth climbs to approximately 10.8 per 10,000 ongoing pregnancies by 42 weeks of gestation.
Comparing the risk of expectant management versus delivery shows a measurable shift after full term. At 38 weeks, the risk of continuing the pregnancy may be comparable to the risk of delivery at that time. By 39 weeks and especially 40 weeks, the statistical mortality risk of continuing the pregnancy is higher than the risk of delivering the baby. This week-by-week increase is a primary reason healthcare providers discuss the timing of delivery as the pregnancy progresses past the due date.
Primary Medical Contributors to Stillbirth
Stillbirth is often a complex outcome resulting from one or more underlying medical mechanisms. The placenta is a common factor, as conditions like placental insufficiency or abruption can disrupt the transfer of oxygen and nutrients to the fetus. Placental failure, where the organ does not function effectively, is linked to a significant percentage of late stillbirths.
Fetal growth restriction (FGR) is another major contributor, where a fetus does not achieve its full growth potential, often due to poor placental function. Other structural issues, such as abnormalities with the umbilical cord, including true knots or constricted blood flow, can also lead to fetal death.
Maternal health conditions can also directly increase the likelihood of stillbirth. These include chronic conditions such as pre-eclampsia, uncontrolled diabetes, and high blood pressure. Furthermore, certain infections, like syphilis or specific bacterial infections, can cross the placenta and cause fetal compromise.
Monitoring and Mitigating Risk
Proactive fetal surveillance is a primary strategy used to mitigate stillbirth risk, especially in pregnancies identified as high-risk due to maternal or placental conditions. One common tool is the Non-Stress Test (NST), which monitors the fetal heart rate for accelerations in response to movement. A reactive NST provides reassurance of immediate fetal well-being and is a measure of adequate oxygenation.
Another common assessment is the Biophysical Profile (BPP), which combines the NST with an ultrasound to evaluate four parameters: fetal breathing, movement, muscle tone, and amniotic fluid volume. A modified BPP often combines the NST with only an assessment of the amniotic fluid index, which can indicate long-term placental function. These tests are typically initiated in high-risk pregnancies starting around 32 to 34 weeks of gestation.
Parental awareness of fetal movement is an important component of risk mitigation. A sudden, sustained decrease in the frequency or strength of movements can be an early sign of fetal compromise and should prompt immediate medical evaluation. When the statistical risk of stillbirth increases, such as when a pregnancy extends past 40 weeks or when test results are non-reassuring, healthcare providers may recommend induction of labor. This decision is made by carefully balancing the risks of continuing the pregnancy against the risks of early delivery.